In an electrophotographic process, a toner is deposited on an electrostatic latent image formed on a photoreceptor made of a photoconducting substance by a magnetic brush development process or the like. The toner image on the photoreceptor is transferred to a transfer material such as paper and a plastic film, and then fixed under heat or pressure or with a solvent to obtain a permanent image. It is thus necessary that the various steps meet various requirements to obtain copied matter.
In particular, in order to obtain a copied image having a high quality, it is necessary that a uniform transfer electric field be formed in the vicinity of the transfer material at the transfer step. Corotron system has been widely used because of its simple mechanism and low cost.
As mentioned above, the corotron system is advantageous in that it requires a simple mechanism and low cost. However, the corotron system is disadvantageous in that it produces ozone upon discharge. Ozone not only is so harmful to human beings that it is placed under strict control but also stains the photoreceptor, causing troubles such as blank areas. The corotron system further has various disadvantages. For example, the corotron system requires a high voltage power supply. This system requires maintenance. For example, deposits such as toner and silicone oil and discharge products must be cleaned off at regular intervals. When the circuit is disconnected, parts must be replaced. Accordingly, a transfer system by a bias roll which causes no production of ozone and requires no maintenance and allows operation at a low voltage has been studied.
In this transfer system, the transfer material is brought into contact with the bias roll to form a transfer electric field. However, when an excess pressure is applied across the transfer material and the bias roll, it is also applied to the photoreceptor and even to the toner image on the photoreceptor. This causes the grains in the toner image to be agglomerated or the toner image to be fixed onto the photoreceptor, inhibiting or disabling the transfer of the toner image to the transfer material. This results in defects in the image transferred to the transfer material, i.e., "blank areas".
In the ordinary development process, a line image has a thicker toner layer towards the center thereof while a solid image has a thicker toner layer towards the edge thereof. Accordingly, blank areas can easily occur in the central part of a line image and in the edge of a solid image or vicinity thereof.
The degree of generation of this phenomenon also depends on the thickness or surface properties of the transfer material. In other words, if the transfer material has a great thickness, the toner image on the photoreceptor undergoes increased pressure and hence tends to suffer from agglomeration and adhesion. If the surface smoothness of the transfer material is high, the adhesivity between the toner grains and the transfer material such as OHP sheet is reduced, causing the generation of blank areas.
Even if the foregoing problems of image quality in the transfer step can be avoided, various problems in the subsequent fixing step must be solved.
In the fixing step, heat-fusing methods have been most frequently employed. These methods can be roughly divided into two types, i.e., contact type system and non-contact type system. In particular, the contact type heat roll fixing system exhibits a good thermal efficiency that allows a high speed fixing. Thus, this system has been widely used in commercial copying machines, printers, etc. in recent years.
However, this heat roll fixing system, too, has some disadvantages. A particularly important disadvantage of this system is that the required amount of energy, i.e., power is rather greater than that of pressure roll fixing system.
Of course, the strength of the fixed image against the transfer material such as paper is far greater in the heat roll fixing system than in the pressure roll fixing system. Further, the heat roll fixing system is also excellent in the prevention of deformation or wrinkle of paper under pressure. It has thus been studied how the power consumption can be reduced in the heat fixing system, that is, how the lowest temperature required for the fixing of the toner can be lowered.
A method useful for the accomplishment of this object includes a method which comprises the use of a toner binder resin having a Tg (glass transition temperature) of scores of degrees lower than that of ordinary binder resin or a toner binder resin having a low molecular weight. However, most of these toners have a fatal defect that it can easily suffer caking or agglomeration during storage or in copying machines.
As a countermeasure for solving the foregoing defect, a method is known which comprises attaching a particulate material such as finely pulverized colloidal silica, alumina or titania to the surface of the toner in an attempt to improve the anti-blocking properties and fluidity of the toner. This countermeasure can provide some improvements in the anti-blocking properties and fluidity without raising the lowest fixing temperature too much and thus seems to be effective. However, it was found that these particulate materials, even if heated to be fused to the surface of the toner, can be easily liberated from the surface of the toner and then exert adverse effects on the photoreceptor, particularly on a photoreceptor coated with an organic polymer or the like. In other words, these particulate materials can be semipermanently fixed to the surface of the photoreceptor after repeated use, causing image defects. Accordingly, this countermeasure cannot be a fundamental solution to the foregoing problems.
Moreover, if a toner comprising a binder resin as described above is used in the heat roll fixing system, it is attached to the heat roll due to its thermal properties, causing an offset phenomenon that stains subsequent copied matter.
Furthermore, in the heat roll fixing system, the fixing roll part is equipped with a peeling claw for preventing the transfer material, paper in general, from being wound on the fixing roll after passing over the fixing roll. However, with the recent rise in the operating speed of copying machines, greater stress is applied to this portion, causing troubles such as release failure and image defects at the front end of the transfer material developed by the peeling claw upon release.
There is a case where a copied image is used as an original to effect further copying. When the copied image is fed into the automatic original feeding apparatus in the copying machine, the original is rubbed by the paper feeding roller in the copying machine to blur or stain the image on the original. In the case of a double-sided original or multi-color original, the surface of the image is rubbed by the paper-feeding roller when it is fed for second copying after the fixing of a first copied image, resulting in the generation of blur or stain on the image. There is a case where a stack of a plurality of originals which has been temporarily stored in a copying machine is withdrawn for second copying one by one by means of a paper feeding roller. In the transfer of these originals, too, the back side of an original is rubbed with the surface of the underlying original to cause rubbing stain or blur on both the two images, resulting in the deterioration of image quality.
In order to eliminate these difficulties, the incorporation of a low molecular weight polypropylene or polyethylene as a lubricant component in a toner has been proposed (JP-B-52-3304 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-57-52574, JP-A-60-151650 (the term "JP-A" as used herein means an "unexamined Japanese patent publication")).
The toner comprising a lubricant as described above incorporated therein can exert some but insufficient effect of enhancing the anti-offset properties against poor releasability from a heat roll, inhibiting scratches by a peeling claw and enhancing the anti-rubbing strength of fixed image. Further, since this toner has a low compatibility between the polyolefin lubricant and the resin component, a large domain is formed therein, resulting in the marked deterioration of the powder fluidity and agglomeration thereof.
As a method for overcoming these problems, some methods are known such as a method which comprises grafting a polyolefin onto a resin so that it is compatibilized in the resin (JP-A-60-457, JP-A-60-93456, JP-A-60-93457) and a method which comprises the use of a modified polyolefin to disperse the lubricant in the resin (JP-A-58-63947, JP-A-59-177570, JP-A-60-3644, JP-A-62-148508, JP-A-63-191817). These methods can provide an enhanced dispersion of polyolefin, making it possible to somewhat inhibit the deterioration of powder fluidity and agglomeration. However, these methods are disadvantageous in that the effect of enhancing the releasability, which is originally required, is lost.
Further, a method which comprises externally adding a silicone oil- or silicone varnish-treated finely pulverized powder to a toner comprising the foregoing lubricant to inhibit the generation of blank areas has been proposed against the foregoing difficulties encountered when an image made of such a toner is transferred using a bias roll (JP-A-3-121462).
This method can exert its effect in the initial stage. However, this method leaves something to be desired in its effect after a prolonged use. For example, this method is subject to the generation of blank areas, particularly when ordinary paper is used under high-temperature and high-humidity conditions or an OHP sheet is used under low-temperature and low-humidity conditions. Further, this method constitutes control by an external additive and thus provides no fundamental improvements in the toner itself.